1. Field of the Invention
The present invention relates to an antenna apparatus. In particular, the present invention relates to a cylindrical antenna apparatus.
2. Description of the Related Art
In recent years, a digital radio receiver that receives a satellite wave (an electric wave from an artificial satellite) or a ground wave (an electric wave on ground) so that a digital radio broadcasting can be received, has been developed. Such a digital radio receiver is generally provided in a movable body such as an automobile and can receive an electric wave having a frequency band of about 2.3 GHz so that radio broadcasting can be received. An antenna apparatus for receiving an electric wave having a frequency band of about 2.3 GHz includes, for example, a flat-type antenna (e.g., patch antenna) or a cylindrical antenna (e.g., loop antenna, helical antenna). An antenna apparatus is designed to be connected to a digital radio tuner via a cable and a connector.
A helical antenna is provided by surrounding a circular or columnar insulating member with at least one conducting wires in a helical manner and can receive a circular polarized wave (e.g., satellite wave) efficiently (see Patent Publication 1 for example). It is preferred that the number of conducting wires is larger in order to improve the receiver sensitivity. However, it is difficult to surround a circular or columnar insulating member with a plurality of conducting wires with a high accuracy.
Therefore, a technique for manufacturing a helical antenna by printing an antenna pattern having a plurality of conducting wires on one surface of a flexible dielectric material film and by forming the film in a cylinder shape, has been suggested (see Patent Publication 2 for example). In the case of such a helical antenna, a plurality of satellite waves (circular polarized waves) received by a plurality of conducting wires are synthesized by phase shift by a phase shifter to have an identical phase. Then, the synthesized satellite wave is amplified by a low noise amplifier (hereinafter referred to as “LNA”) and is sent to a receiver. Specifically, a combination of the helical antenna, the phase shifter, and the LNA constitute an antenna apparatus.
The present inventors also have developed an antenna apparatus 1 in which a substrate on which an electronic part (e.g., LNA) is mounted is provided on the inner side of a helical antenna to reduce the length of the antenna apparatus in the axial direction (see FIG. 1). Such an antenna apparatus includes a tubular flexible dielectric material film in which an antenna pattern is printed on one surface; a substrate 50 on which an LNA is mounted and which is positioned on the inner side of the dielectric material film; and a tubular outer packaging case for covering the dielectric material film.
As shown in a development view of the dielectric material film (see FIG. 4), one surface of the dielectric material film includes a helical antenna section 12 on which an antenna pattern comprising a plurality of conducting wires is printed; and a phase shifter section 13 that is formed in a tubular shape and that functions as a phase shifter. When the dielectric material film is formed in a tubular shape, the helical antenna section 12 is positioned at the upper part in the axial direction and the phase shifter section 13 is positioned at the lower part thereof. On the lower end of the phase shifter section 13, an output terminal 16 of the phase shifter is provided. On the inner side of the tubular dielectric material film, a substrate 50 on which an LNA 20 electrically connected to an output terminal 16 and a cable 2 is mounted, is provided to extend along the axial direction.
[Patent Publication 1] Japanese Laid-Open Publication No. 2001-339227
[Patent Publication 2] Japanese Laid-Open Publication No. 2003-37430
However, in the conventional antenna apparatus, the output terminal 16 of the phase shifter is provided on the lower end of the tubular dielectric material film. Therefore, an input unit 51 for inputting a signal from the phase shifter at the lower part of the substrate 50 is required (see FIG. 9). Furthermore, because the cable 2 is provided at the lower part of the antenna apparatus, an output unit 52 for outputting a signal to the cable 2 also must be provided at the lower part of the substrate 50. Therefore, there was a problem where because the input unit 51 for inputting a signal from the phase shifter is adjacent to the output unit 52 for outputting a signal to the cable on the substrate 50, the isolation between an input and an output is not sufficient.